AIM: We aimed to grasp the actual working hours of Japanese obstetricians and gynecologists (OB/GYN doctors) as accurately as possible, using the same method of the Ministry of Health, Labour, and Welfare (MHLW). METHODS: The time study targeted OB/GYN doctors working at 10 universities nationwide including Niigata University and 21 institutions which take a role of perinatal care in Niigata prefecture. Working hours per week were calculated based on the following categories: regular and overtime work inside the hospital, work outside the hospital, self-improvement, education, research, and others. Data on weekly working hours were converted to yearly data for analyses. RESULTS: A time study of 10 universities nationwide revealed that 30% of doctors work overtime for more than 1860 h even if they do not include on-call shifts in their working hours. In 21 institutions in Niigata, physicians in Niigata University worked more overtime than other hospitals. It became clear that community health care was supported by dispatching physicians working at university. Furthermore, the results of simulations predicted the pessimistic situation of perinatal medical care in Niigata. CONCLUSIONS: Our study showed the possibility to exist much more OB/GYN doctors who work more than 1860 h of overtime work per year than the data presented by the MHLW based on nation-wide survey in 2019. The fact that the working hours at the side jobs had a great influence on the increase in overtime work of physicians in University was the same result as the report of MHLW published in 2021.